Ceramic membranes are made from inorganic materials such as, alumina, titania and zirconia oxides including mixtures thereof and have benefits, especially compared to polymeric membranes, in view of their characteristics. They are chemically inert and feature high mechanical, thermal and hydrothermal stabilities. Ceramic membranes are known to be robust in extreme processing conditions such as e.g. temperature, corrosion or cleaning conditions and exhibit long lifetimes. Therefore ceramic membranes are suitable for being used in processes where thermal, mechanical and hydrothermal stability are required as well as in those applications where chemical resistance is necessary.
Ceramic membranes have their own surface chemistry essentially consisting of M1-OH and M1-O-M1 structure in which M1 is a transition metal or a metal. In view of such surface chemistry, a hydrophilic behaviour of ceramic membranes limits their applications. By means of chemical surface modification, also denoted as functionalisation, the character of the membrane can be changed, for instance from hydrophilic to hydrophobic. Surface modification reactions involve the replacement of OH groups provided on the surface of the membrane by other groups, e.g. organic functional groups, in order to give the membranes a specific character such as e.g. hydrophobicity, but also other functionalities such as for instance selective adsorption sites, anchoring positions for immobilization, chiral sites etc.
Various methods have been reported for the surface-modification of ceramic membranes including methods involving for instance co-condensation reactions, grafting reactions with organosilane or phosphonic acids, polymerization reactions on the surface etc.
For instance, WO 99/61140 discloses that by co-condensation of a hydrocarbyl metal alkoxide with a sol-gel precursor such as e.g. a metal alkoxide, a hydrophobic sol can be obtained. This sol is then coated on a membrane support. Co-condensation refers to a process where the functionalisation of the membrane occurs during the synthesis step. Additional organosilane precursors are used in the synthesis together with the normal silica or metal oxide (e.g. metal alkoxide) precursor. During the synthesis step, both precursors undergo the sol-gel process and condensate together to form a homogeneous hybrid sol that can be coated on the membrane support. Co-condensation incorporates the functional groups during synthesis such that the modifications are not concentrated at the surface such as in a post-modification method. This technique of co-condensation has some important disadvantages. The number/concentration of organic functional groups that can be applied on the membrane is limited, and introduction of high concentrations of organic functional groups would seriously decrease the structural properties and stability of the formed membranes. In addition, the number of possible precursor molecules that can be added during the condensation reaction is limited and such molecules are often very expensive. Therefore, the versatility of such co-condensation method is limited. The stability of these materials towards hydrolysis reactions may, in some cases, be higher. However, due to the lower number of functional groups on the surface of the membrane, these membranes show less functionality, there is no control with regard to the position of the functional groups in the membrane and the membranes have a lower Q4/Q3 ratio, leading to a lower general stability of the membrane.
An alternative approach for the preparation of functionalized membranes consists of applying surface grafting reactions. Organosilane grafting is one of the applied techniques. US application number 2006/237361 for instance discloses a method for the impregnation of a ceramic membrane with an organosilane agent. The organosilane agent is of general formula R1R2R3R4Si in which at least one R group is a hydrolyzable group and at least one R group is a non-hydrolyzable group like alkyl group, phenyl group, which can be at least partially fluorinated. Bonding of the organosilane agent to the membrane surface occurs by a condensation reaction of the hydrolyzable groups with OH groups on the surface of the oxide membrane. This results in covalent bonding of the organosilane agent on the membrane through an oxygen bridge which is very susceptible to hydrolysis. Moreover when organosilane grafting is applied on metal oxide membranes comprising TiO2, ZrO2 low stabilities are obtained, which may ultimately result in unwanted leaching of organic functional group(s) from the membrane after some time on stream.
U.S. Pat. No. 6,596,173 discloses the grafting of filtration membranes with organomineral compounds. These organomineral compounds react via their hydrolysable group(s), i.e. their alkoxy or carboxyl function(s), with the mineral functions of the separating membrane layer. Whereas the resulting M-O—R bond is a covalent bond, the oxygen makes the grafted material unstable and easily hydrolysable. As a result thereof the organomineral groups are removed easily from the membrane over time thereby rendering the filtration membrane less efficient. The same leaching of organic functional group(s) from the membrane occurs in membranes as disclosed in DE 102 23 103. This German patent application discloses a similar grafting technique with sol-gel precursors, the resulting membrane having similar drawbacks as the membranes according to U.S. Pat. No. 6,596,173.
Grafting with phosphonic acids is another approach for the formation of hydrophobic or functional ceramic materials. This method involves a coordination or iono-covalent interaction of a phosphonic acid with a metaloxide surface (J. Caro, M. Noack, P. Kölsch, Micropor. Mesopor. Mater. 22 (1998) 321). However, leaching problems of the organic functional groups are likely to happen, depending on the type of solvent used and at high flux rates, since the complexes are sensitive to a nucleophilic attack. In addition, phosphor is known to have negative influence on the environment. Moreover, the amount of available organic functional groups on phosphonic acids is limited.
In view of the above, although methods are available in the prior art for the surface modification of ceramic membranes, these methods are limited in various ways, e.g. towards modification with different types, amounts of organic functional groups applied, practicability of the methods, etc.
Furthermore, the surface-modified ceramic membranes that can be obtained with above-disclosed methods sometimes show an inadequate thermal and/or hydrothermal stability. More in particular, an important problem of prior art modified ceramic membranes is that they sometimes show considerable release (leakage) of bond organic functional groups, especially under harsh operational conditions.
In view of the above drawbacks, it is an object of the present invention to provide a method for preparing an organic functionalized matrix, and in particular an organic functionalized ceramic membrane, which overcomes at least some of the above mentioned problems. More in particular, the invention aims to provide a method wherein a surface of an inorganic matrix or of a ceramic membrane has been modified by covalently binding an organic functional group on said surface directly on the metal M1. In particular, the present invention aims to provide a method which is highly versatile, allowing a broad variety of modifications of surfaces of matrices or membranes.
The invention also aims to provide an organic functionalized matrix, i.e. a matrix of which a surface has been modified with organic functional groups, and in particular aims to provide a organic functionalized ceramic membrane that has adequate thermal and/or hydrothermal stability and that shows poor or substantially reduced leaching of attached groups. Another object of the invention is to provide an organic functionalized matrix, and in particular an organic functionalized ceramic membrane, which can be modified in a controlled manner and which has a high modification degree.